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arXiv · 0911.1356

Star formation rates and stellar masses of z=7-8 galaxies from IRAC observations of the WFC3/IR ERS and the HUDF field

I. Labbe·V. Gonzalez·R. J. Bouwens·G. D. Illingworth·M. Franx·M. Trenti·P. A. Oesch·P. G. van Dokkum·M. Stiavelli·C. M. Carollo·M. Kriek·D. Magee
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Abstract

We investigate the Spitzer/IRAC properties of 36 z~7 z-dropout galaxies and 3 z~8 Y-dropout galaxies derived from deep/wide-area WFC3/IR data of the Early Release Science, the ultradeep HUDF09, and wide-area NICMOS data. We fit stellar population synthesis models to the SEDs to derive mean redshifts, stellar masses, and ages. The z~7 galaxies are best characterized by substantial ages (>100 Myr) and M/L_V~0.2. The main trend with decreasing luminosity is that of bluing of the far-UV slope from beta~-2.0 to beta~-3.0. This can be explained by decreasing metallicity, except for the lowest luminosity galaxies (0.1 L*{z=3}), where low metallicity and smooth SFHs fail to match the blue far-UV and moderately red H-[3.6] color. Such colors may require episodic SFHs with short periods of activity and quiescence ("on-off" cycles) and/or a contribution from emission lines. The stellar mass of our sample of z~7 star forming galaxies correlates with SFR according to log M* = 8.70 (+-0.09) + 1.06(+-0.10) log SFR, implying star formation may have commenced at z>10. No galaxies are found with SFRs much higher or lower than the past averaged SFR suggesting that the typical star formation timescales are probably a substantial fraction of the Hubble time. We report the first IRAC detection of Y-dropout galaxies at z~8. The average rest-frame U-V=0.3 (AB) of the 3 galaxies are similar to faint z~7 galaxies, implying similar M/L. The stellar mass density to M_{UV,AB}<-18 is rho*(z=8) = 1.8^{+0.7}_{-1.0} x 10^6 M_sun Mpc^-3, following log rho*(z)= 10.6(+-0.6) - 4.4(+-0.7) log(1+z) [M_sun Mpc^-3] over 3<z< 8.

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I. Labbe, V. Gonzalez, R. J. Bouwens, G. D. Illingworth, M. Franx, M. Trenti, P. A. Oesch, P. G. van Dokkum, M. Stiavelli, C. M. Carollo, M. Kriek, D. Magee. 2010-05-24. Star formation rates and stellar masses of z=7-8 galaxies from IRAC observations of the WFC3/IR ERS and the HUDF field. https://doi.org/10.1088/2041-8205%2F716%2F2%2Fl103

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